1. Field of the Invention
The present invention generally relates to optical polarizers, and more particularly to a novel liquid crystal display panel and an overhead projector which incorporate tuned cholesteric polarizers.
2. Description of the Prior Art
Liquid crystal display (LCD) panels generally employ one of several different types of polarizers, including dyed stretched polymeric (neutral and dichroic), cholesteric or thin-film polarizers. For example, U.S. Pat. No. 4,917,465 discloses an LCD panel (depicted in FIG. 1) which uses both neutral polarizers 1 and dichroic polarizers 2 in a stacked arrangement. The dichroic dyes and associated liquid crystal layers 3 are selected to provide desired optical responses in order to yield the three primary subtractive colors, magenta, cyan and yellow.
One drawback to conventional dyed polarizers is that they are absorptive, i.e., polarization occurs due to different absorption coefficients for light polarized in different directions. This results in a loss of more than 50% of the available light, which adversely affects illumination in display systems using LCD's. Absorption of this amount of energy can also lead to heat management problems. In some projection systems using LCD's, such as overhead projectors (OHP's), illumination difficulties may be overcome by providing a brighter light source, such as a metal halide lamp; this solution, however, is expensive, and typically exacerbates heat management problems, and further introduces a serious safety hazard.
Another drawback to dichroic dyed polarizers is that their spectral cutoffs are limited by the dyes themselves. In many uses, such as stacked, color supertwisted nematic (STN) LCD panels, these cutoffs are not as sharp as would be desired, and their use results in less than ideal color purity and brightness or, alternatively, necessitates use of neutral polarizers instead.
Cholesteric polarizers have sharper spectral cutoffs which can be employed to improve these uses, and are reflective rather than absorptive but, prior to the present invention, cholesteric polarizers have not been incorporated into flat panel LCD designs. These polarizers utilize liquid crystal material in which the elongated molecules are parallel to each other within the plane of a layer, but the direction of orientation twists slightly from layer to layer to form a helix through the thickness of the polarizer. Cholesteric polarizers are either right- or left-handed and, depending upon this handedness, transmit or reflect circularly polarized light in a narrow optical bandwidth. As suggested by Schadt et al. (see SID 1990 Symposium Digest pp. 324-326; and 29 Jap. J.Appl.Phys. pp. L634-L637 and pp. 1974-1984), the reflective properties of cholesteric filters may be exploited to recycle the reflected light. See also Maurer et al. (SID 1990 Symposium Digest pp. 110-113, and 1991 Record of the International Display Research Conference, pp. 57-59). Unfortunately, these devices are incompatible with the configuration of a flat LCD panel, such as is used in either direct view or with an OHP since, for example, their wide-aperture case does not generate highly polarized light; other of the devices are particularly unsuitable in stacked LCD designs.
In U.S. Pat. No. 4,987,410, cholesteric polarizers are used to selectively block one of two images in a head-up/head-down display. While that patent further discloses the use of multiple polarizers to selectively block discrete color images, the polarizers are never used for manipulating the color tone of a single image. The configuration of the '410 device is also generally incompatible with that of an LCD panel. See additionally U.S. Pat. No. 5,050,966. It would, therefore, be desirable and advantageous to devise an LCD panel which incorporates cholesteric polarizers, but provides high efficiency, wide aperture transmission of the light for stacked LCD panel designs. It would be further advantageous if such a wide aperture polarized light source could be directly incorporated into an OHP to accommodate any type of LCD panel (e.g., color or monochrome). Finally, the systems would preferably impart greater brightness, and be usable over a wider range of operating temperatures.